A hammer drill is a rotary drill with a hammering action. The hammering action provides a short, rapid hammer thrust to pulverize relatively brittle material and provide quicker drilling with less effort. Lower power units are usually titled as hammer drills. It also called rotary hammer. It is well suited for drilling holes in masonry or stone. It is also used to drill holes in concrete footings to pin concrete wall forms and to drill holes in concrete floors to pin wall framing. It can either be a drill or it can be a replacement chuck that is installed on a power drill. When the drill is set to the hammer drill position it allows the chuck to move backward and forward over a short distance, allowing a spring to be compressed. When the spring is compressed, the chuck assembly is pushed as far back as it can go against a small eccentric flange.